Conventionally, in the art of motor control, signals from various sensors in a vehicle are converted from Analog to Digital (A/D) conversion, and the A/D converted voltage is inputted to a controller for controlling various in-vehicle devices.
For example, a voltage of an external power source, which corresponds to a battery voltage, is A/D-converted, and, based on an A/D conversion value, a final injection time of a fuel injection valve is determined by a control device in a patent document, JP H06-350450 A (patent document 1).
In general, when a reference voltage at the time of A/D conversion is lower than the normal value, an A/D conversion may be falsely performed, i.e., may not yield an accurate A/D conversion result.
In the control device of the patent document 1, the regulator generates two internal power source voltages (i.e., 5 V, 2.5 V), of high and low, from the battery voltage, for switchingly using one of the two voltages as a reference voltage at the time of A/D conversion depending on operational situations.
In the control device of the patent document 1, when performing the A/D conversion of a voltage that is not in a proportional relationship with the internal power source voltage, e.g., a battery voltage, a heater current voltage, etc., it is first determined whether the battery voltage is higher than a predetermined voltage value (5 V), and, upon determining that the battery voltage is higher than the predetermined voltage value, the A/D conversion is performed by using the higher one of the two reference values (5 V).
On the other hand, when it is determined that the battery voltage is lower than the predetermined voltage value, the A/D conversion is performed by using the lower one of the two the reference voltages (2.5 V).
Thereby, even when the external power source voltage falls, the A/D conversion of the voltage corresponding to the battery voltage, the heater current voltage, etc. is accurately performable.
However, in the control device of the patent document 1, when the battery voltage is lower than the predetermined voltage value (e.g., a voltage between 2.5 V and 5 V), the A/D conversion is performed with a lower reference voltage (i.e., 2.5 V) that is lower than a usual reference voltage (i.e., 5 V), the Signal-to-Noise (S/N) ratio gets worse and may possibly lead to an inaccurate A/D conversion.
Therefore, while the battery voltage falls to a lower-than-normal value (e.g., 2.5 to 5 V) relative to the predetermined voltage value, various in-vehicle devices may not be highly or accurately controllable by the control device of the patent document 1. Further, when the battery voltage is equal to or lower than the lower reference voltage (2.5 V) or the like, the reference voltage falls to be lower than the normal value (2.5 V), and the A/D conversion may not be accurately performable.
Further, in case that the control device of the patent document 1 is applied to, for example, a control of an electric power steering device of an idle stop vehicle, when an engine restarts after the idol stop, the battery voltage may fall to be equal to or lower than the predetermined voltage value by the rotation of the starter motor, and the control of the electric power steering device may not be performable with high accuracy.